Electroactive polymers (EAP) are an emerging class of materials within the field of electrically responsive materials. EAP's can work as sensors or actuators and can easily be manufactured into various shapes allowing easy integration into a large variety of systems.
Materials have been developed with characteristics such as actuation stress and strain which have improved significantly over the last ten years. Technology risks have been reduced to acceptable levels for product development so that EAP devices are commercially and technically becoming of increasing interest. Advantages of EAP devices include low power, small form factor, flexibility, noiseless operation, accuracy, the possibility of high resolution, fast response times, and cyclic actuation.
The improved performance and particular advantages of EAP material give rise to applicability to new applications.
An EAP device can be used in any application in which a small amount of movement of a component or feature is desired, based on electric actuation. Similarly, the technology can be used for sensing small movements.
The use of EAP devices enables functions which were not possible before, or offers a big advantage over common sensor/actuator solutions, due to the combination of a relatively large deformation and force in a small volume or thin form factor, compared to common actuators. EAP devices also give noiseless operation, accurate electronic control, fast response, and a large range of possible actuation frequencies, such as 0-20 kHz.
Devices using electroactive polymers can be subdivided into field-driven and ionic-driven materials.
Examples of field-driven EAP devices are dielectric elastomers, electrostrictive polymers (such as PVDF based relaxor polymers or polyurethanes) and liquid crystal elastomers (LCE).
Examples of ionic-driven EAP devices are conjugated polymers, carbon nanotube (CNT) polymer composites and Ionic Polymer Metal Composites (IPMC).
Field-driven EAP devices are actuated by an electric field through direct electromechanical coupling, while the actuation mechanism for ionic EAP devices involves the diffusion of ions, and they are hence current driven devices. Both classes have multiple family members, each having their own advantages and disadvantages.
FIGS. 1 and 2 show two possible operating modes for an EAP device.
The device comprises an electroactive polymer layer 14 sandwiched between electrodes 10, 12 on opposite sides of the electroactive polymer layer 14.
FIG. 1 shows a device which is not clamped. A voltage is used to cause the electroactive polymer layer to expand in all directions as shown.
FIG. 2 shows a device which is designed so that the expansion arises only in one direction. The device is supported by a carrier layer 16. A voltage is used to cause the electroactive polymer layer to curve or bow.
The nature of this movement for example arises from the interaction between the active layer which expands when actuated, and the passive carrier layer. To obtain the asymmetric curving around an axis as shown, molecular orientation (film stretching) may for example be applied, forcing the movement in one direction.
The expansion in one direction may result from the asymmetry in the electroactive polymer, or it may result from asymmetry in the properties of the carrier layer, or a combination of both.
In certain applications, an array of actuators can be useful, for instance in positioning systems and controlled topology surfaces. However, as the driving voltages of the actuators are fairly high it quickly becomes expensive to drive each actuator individually with its own driver IC.
A passive matrix array is a simple implementation of an array driving system using only row (n rows) and column (m columns) connections. As only (n+m) drivers are required to address up to (n×m) actuators, this is a far more cost effective approach—and also saves cost and space of additional wiring.
Ideally, in a passive matrix device, each individual actuator should be actuated up to its maximum displacement without influencing the adjacent actuators. However, in arrays of traditional EAP actuators (without any voltage threshold behavior) some cross talk to adjacent actuators will be present. When a drive voltage, for example, is applied to actuate one actuator, the actuators around it also experience a voltage and will partially actuate, which is an unwanted effect for many applications. Hence, with a passive matrix addressing scheme it is not straightforward to individually address each actuator independently of the others.
The use of an active matrix for addressing arrays of electroactive polymer actuators has been contemplated, for example for electronic braille applications. An active matrix approach involves providing a switching device at each electroactive polymer actuator, at the intersection of a row conductor and a column conductor. In this way, each actuator in the array can—if desired—be individually actuated. An active matrix addressing scheme means it is possible to have any random pattern of actuators in the array actuated at the same time.
When designing an active matrix design for a field driven EAP, a problem arises that the switching device, for example transistor, needs to be able to withstand high actuation voltages, which may be hundreds of volts. This is far above the possible voltages which can be handled by existing transistors suitable for integration into an array device.
There is therefore a particular interest in ionic (current driven) EAP devices for use with an active matrix addressing scheme. Ionic EAP devices are activated by an electrically induced transport of ions and/or solvent. They usually require low voltages but high currents, for example they may operate at low and hence safer voltages of around 5V. They require a liquid/gel electrolyte medium (although some material systems can also operate using solid electrolytes). The ability to address at lower voltages enables use of readily available switching devices.
The different types of ionic EAP device mentioned above will now be discussed in further detail.
FIG. 3 shows an example of an Ionic Polymer Metal Composites (IPMC) EAP, comprising a polymer membrane 30 between conducting electrode surfaces 32. Anions 34 are fixed in the membrane and cations 36 are mobile. The cations 36 are hydrated by water molecules 38. When a voltage is applied, the hydrated cations migrate to the cathode, leading to polymer expansion.
The IPMC actuator in this way consists of a solvent swollen ion-exchange polymer membrane laminated between two thin metal- or carbon based electrodes and requires the use of an electrolyte. Typical electrode materials are Pt, Gd, CNTs, CPs, Pd. Typical electrolytes are Li+ and Na+ water based solutions. When a field is applied and a current induced, cations typically travel to the cathode side together with water. This leads to reorganization of hydrophilic clusters and to polymer expansion. Strain in the cathode area leads to stress in the rest of the polymer matrix resulting in bending towards the anode. Reversing the applied voltage and inducing a current in the opposite direction inverts bending. Well known polymer membranes are Nafion (trade mark) and Flemion (trade mark).
FIG. 4 shows an example of a conjugated polymer actuator, comprising an electrolyte 40 sandwiched by two layers 42, 44 of the conjugated polymer. The electrolyte is used to change oxidation state. When a potential is applied to the polymer through the electrolyte, electrons are added to or removed from the polymer by the induced current, driving oxidation and reduction. Reduction results in contraction, oxidation in expansion. Thus, curvature is induced towards the reduced side 46 as shown in FIG. 4. In some cases, thin film electrodes are added when the polymer itself lacks sufficient conductivity. The electrolyte can be a liquid, a gel or a solid material (i.e. complex of high molecular weight polymers and metal salts). Most common conjugated polymers are polypyrolle (PPy), Polyaniline (PANi) and polythiophene (PTh).
FIG. 5 shows an example of a Carbon Nano Tube (CNT) actuator, in which a carbon nano tube 48 is suspended in an electrolyte 49. The electrolyte forms a double layer with the nanotubes, allowing injection of charges. This double-layer charge injection is considered as the primary mechanism in CNT actuators. The CNT acts as an electrode capacitor with charge injected on the CNT, which is then balanced by the electrical double-layer formed by movement of electrolytes to the CNT surface. Changing the charge on the carbon atoms results in changes of carbon-carbon bond length. As a result, expansion and contraction of single CNT can be observed.
Other examples include ionic polymer gels. Note that the electrodes used can be continuous, or segmented.
For low cost applications, it is desired to use low cost transistor technology, for example amorphous silicon transistors. These, as well as other low cost and low voltage technologies, typically have poorer stability, for example they suffer threshold voltage drift, which makes their usage difficult, and presents problems for the design of the driving circuitry.